Technical Abstract:
The present study develops sex-specific equations to predict total body water (TBW) and fat-free mass (FFM) from bioelectric impedance (BIA). These equations facilitate the estimation of body composition in the Third National Health and Nutrition Examination Survey (HNANES III, 1998-1994) in which BIA measures are available. The validation sample (N=1842) consisted of data sets from new York City, ND, and OH. The independent cross-validation sample (N-521) consisted of data set from Chicago, Maryland, Ohio. The subjects were 12 t 65+ years of age. The dependent variable were FFM and TBW. FFM was derived form body density using a multi-component model including TBW from D20, bone mineral content from DXA and body weight. The independent variables were age, stature, weight, BMI, resistance (R), and stature*2 divided by R (S*2/R). An all possible subset regression was used to select the equations. These prediction equations were cross-validated. For the non-Hispanic White and Black males, the equations included weight, S*2/R, R with R*2 value 0.90 for FFM and weight and S*2/R with an R*2 value of 0.85 for TBW. The corresponding RMSEs were 3.7 kg and 3.6 lifters, respectively. For females, the equations include weight, S*2/R, and R with an R*2 value of 0.85 for FFM and weight and S*2/R with an R*2 value of 0l78 for TBW and RMSEs of 2.8 for FFM and 2.6 liters for TBW respectively. The pure errors form the cross validation ranged from 3.5 to 4.5 kgs for FFM and 3.0 to 4.0 liters for TBW. These BIA prediction equations are applicable to the BIA data in the NHANES III survey to obtain means and standard deviations for FFM, TBW, TBF and %BF for non-Hispanic Whites, Black and Mexican Americans.